Systems employing data storage elements include, for example, video cameras and image displays. Such systems employ an addressing structure that provides data to or retrieves data from the storage elements. One system of this type to which one embodiment of the present invention is particularly directed is a general purpose flat panel display, whose storage or display elements store light pattern data. A flat panel display comprises multiple display elements distributed throughout the viewing area of a display surface. A flat panel display system is desirable because it does not necessarily require a cathode-ray tube to develop a display image. A cathode-ray tube is undesirable because of its size, fragility, and need for high voltage drive circuitry.
One type of flat panel display system employs an addressing structure that accomplishes direct multiplexing of multiple liquid crystal cells or display elements arranged in an array. Each of the liquid crystal cells is positioned between a pair of electrical conductors that selectively apply select and deselect voltage signals across the liquid crystal cell to change its optical properties and thereby change the brightness of the image it develops. A display system of this type is characterized as "passive" because no "active" electronic device cooperates with the liquid crystal cell to modify its electro-optical properties. Such a display system suffers from the disadvantage of being capable of implementation with only a limited number of addressable lines (i.e., up to about 250) of video information or data for developing a display image.
One expedient for increasing the number of addressable lines of data in a liquid crystal display system entails the use of an addressing structure in which a separate electronic device cooperates with each liquid crystal cell to increase the effective nonlinearity of its electro-optic response to the select and deselect voltage signals. Some of what are referred to as "two-terminal" device addressing techniques can be characterized in this way. Although increasing the effective nonlinearity of a display element may allow greater multiplexing capabilities in a bilevel display, there remain many difficulties with this technique in respect to achieving gray scale performance.
The objective in designing liquid crystal matrix display systems having full gray scale capability is to provide an addressing structure that does not rely on obtaining the nonlinearity function from the liquid crystal material. An addressing structure that uses a matrix of electrically "active" elements accomplishes this objective by employing at each picture element an electronic switch that is separate from the liquid crystal material. The active matrix uses two- or three-terminal solid state devices in association with each liquid crystal cell to develop the needed nonlinearity and display element isolation. Addressing structures constructed of two-terminal devices can employ diodes of various types, and addressing structures constructed of three-terminal devices can employ thin film transistors (TFT) of various types manufactured from different semiconductor materials.
One problem with two- and three-terminal active matrices is that the very large number of active devices makes it very difficult to fabricate the matrix in large quantities with high production yields. Another problem, which is characteristic of TFT devices, is the difficulty of constructing thin film transistors with sufficiently high "off resistances." A relatively low "off resistance" provides a display element that may not hold the charge developed across it for the requisite time. A relatively low "off resistance" also decreases the "off resistance" to "on resistance" ratio, which preferably exceeds 10.sup.6 to promote proper operation of the TFT matrix. TFT matrices sometimes employ a separate storage capacitor with each display element to offset the effect of an insufficiently high "off resistance." The use of separate storage capacitors increases, however, the complexity of the TFT matrix incorporating them and the likelihood of decreased production yields. One other possible problem with a TFT active matrix is that the size of a TFT can be relatively large compared to that of the display element because "on" current requirements tend to increase the dimensions of a TFT device. This may affect the light efficiency of the device.
An active matrix formed of TFT devices is capable of developing black-and-white and colored images. To develop colored images, the active matrix employs a color filter containing multiple groups of spots in different colors spatially aligned with the display elements. A group of display elements aligned with spots of different colors would, therefore, define a single image pixel.
Flat panel display systems can also be implemented with display elements employing an ionizable gas or plasma that glows to produce on a display surface luminous regions whose color is characteristic of the type of gas used. The luminous regions are selectively activated to form a display image.
Another type of flat panel display system employs a plasma to generate electrons that are accelerated to strike a phosphor and produce a luminescent spot. Such a flat panel display provides increased efficiency in brightness but is difficult to produce with large display areas and requires complex drive circuits. Such flat panel displays can be constructed with multiple electron-excited phosphors having different spectral characteristics to provide multi-colored images.
Problems with gas-plasma flat panel displays are purportedly alleviated through the use of gas-discharge displays of the plasma-sac type. In such displays, a plasma-sac produced on the cathode side of an apertured insulator moves from one aperture to another to effect a raster-type scan. The plasma-sac-type gas-discharge displays are also complex to fabricate and are susceptible to low production yields.